1. Field of the Invention
The present invention is directed to cutting tools and cutting inserts. More particularly, the present invention is directed to ball nose end mills with replaceable cutting inserts.
2. Description of the Invention Background
Most manufactured products contain one or more components manufactured by machining, and often the machining step or steps produce the components to very precise tolerances. Machining, while one of the most basic and important processes used in manufacturing metal products, also is one of the more expensive. Thus, even modest improvements in the machining process may yield substantial cost savings.
One of the basic machining processes is milling. In milling, a form is generated in a workpiece by the action of a rotating cutting tool on the workpiece. End milling is a particular type of milling that normally involves the use of a relatively small diameter cutting tool with one or more cutting edges at its end. Ball nose end milling is a particular type of end milling that uses a cutter with a radiused or arcuate cutting edge at the end of the cutter. Ball nose end milling cutters, commonly called “ball nose end mills”, are ideal for machining 3-dimensional forms in molds and dies and for providing a radiused or arcuate form in the workpiece. Ball nose end mills may be solid with cutting edges ground in the surface or be constructed to use replaceable cutting inserts. The present invention relates to ball nose end mills that use replaceable cutting inserts. It is particularly useful when applied to end mills of that type that have an effective cutting diameter in the range of ⅜″ to 1.0″. As in other machining processes, the design of the cutting tool is critical to the efficiency by which material can be removed from the workpiece. Thus, substantial sums are spent each year to research and develop improved cutting tools for machining.
Ball nose end mills typically have several disadvantages due to the fact that during the machining operation the cutting speed changes along the cutting edge and is zero at the axial center of the tool. This normally results in premature wear and dulling, chipping or breaking of the cutting edge which shortens the useful life of the tool. Prior design tools typically use a cemented carbide replaceable insert that needs to be replaced frequently due to excessive wear or breakage. It has generally been thought that ceramic could not be used as an insert material for ball nose end milling because ceramic must be run at a high surface feed rate to generate the heat required to plasticize the metal being cut during the machining operation. The slow cutting speeds toward the axial center of a ball nose end mill and the zero cutting speed at the center of the tool were thought to be insufficient to generate the required heat. And the brittleness of ceramic was thought to make it more susceptible to edge chipping and cracking during a ball nose end milling operation.
In a typical prior art ball nose end mill, an insert with a radiused or arcuate cutting edge is retained in an axial slot at the end of a cylindrical cutting tool body. The cutting tool insert may be held in place by a number of different means including a screw which draws the sides of the slot together, a clamp or brazing. Examples of prior art ball nose end mills that use replaceable inserts are shown in U.S. Pat. Nos. 5,782,589, 6,158,927, 7,044,695, 5,632,576, 6,231,275, 6,607,333, and U.S. Pat. No. 5,348,426. The present invention is a significant improvement over the prior art in that it allows the use of ceramic inserts in ball nose end milling with effective use of cutting edges to the axial center of the end mill. It also provides improved performance and longer tool life.
The use of ceramic inserts is particularly advantageous because ceramic generally has high hardness and is relatively resistant to oxidation and, therefore, it will exhibit low tool wear at high cutting temperatures. The faster the cutting speeds, the higher the cutting temperature. The properties of ceramics allow ceramic cutting tools to be run at faster cutting speeds while maintaining long tool life, thereby improving the efficiency of the machining process.